1. Field of the Invention
The present invention relates generally to charge pump circuits, and in particular to charge pump circuits that provide a regulated output voltage.
2. Discussion of the Related Art
A charge pump circuit is generally used for boosting a power supply voltage to a higher voltage level to be used for powering ancillary circuits. Closed loop control is sometimes used with charge pumps to provide a regulated output voltage. However, charge pumps that include closed loop control typically have extremely slow response times due to the relatively small amount of energy that can be transferred during each clock cycle. Exacerbating the slow response times is the effect of inherent leakage currents that occur within the charge pump. When diodes within the charge pump are back biased, a small but significant leakage current flows backwards through the diode. The leakage current transfers charge out of the output storage devices back to the voltage supply, causing the output voltage to slowly decrease. The output voltage continues to decrease until the output controller detects an error of sufficient magnitude between the output voltage and the voltage reference. The controller then enables several cycles of energy to be transferred to the output storage devices. However, due to the slow response of the controller, the output voltage will typically overshoot slightly. Then, the cycle of leakage current, decreasing output voltage, slow response controller starts anew. The steady-state effect is a low-magnitude, low-frequency oscillation occurring on the output voltage that is somewhat related to the magnitude of the leakage current. Compensating for the effect of leakage currents prior to manufacturing is problematic, since the magnitude of leakage currents are dependent on several factors including the semiconductor manufacturing process, die defects, and temperature.